Allograft survival with no adverse effects is an ultimate goal in transplantation. Over the past several decades, a large array of immunosuppressive agents has been developed and used for patients. However, immunosuppression does not guarantee the prevention of alloreaction over time in patients who receive organs, tissues, and hematopoietic stem cell (HPSC) transplantation. As a consequence, patients succumb to graft-versus-host disease (GVHD) as well as serious side effects from life-long immunosuppression. Furthermore, controlling GVHD with nonspecific immunosuppression neither spares pre-existing memory cells nor discriminates between alloreactive and non-alloreactive T cells. Thus, although GVHD could be controlled in some degrees by immunosuppression, it is at the cost of increased incidence of graft failure, leukemia relapse, and compromised immunity to post-transplant infections, such as cytomegalovirus (CMV). Therefore, a novel therapeutic strategy that prevents GVHD, while preserving host immunity to infections and graft versus leukemia (GVL) effects will bring great benefit to patients. We have recently found that human dendritic cells (DCs) activated via different lectin-like receptors (LLRs) can program the quality and quantity of antigen-specific T cells in different ways. Of the LLRs tested, DC-asialoglycoprotein receptor (DC-ASGPR) has a unique function to generate antigen-specific Tregs that produce IL-10. Small numbers of such Tregs were sufficient to suppress the same antigen-specific effector T cell proliferation and inflammatory cytokine expression. DCs activated via DC-ASGPR with anti-DC-ASGPR antibody express IL-10, which promotes antigen-specific Treg responses. In addition, anti-DC-ASGPR antibody significantly reduces allogeneic T cell proliferation. Thus, DC-ASGPR could be a novel target to mount alloantigen-specific Tregs in patients without interfering with host immunity to pathogens and GVL effects. In this study, therefore, we propose to investigate the molecular (Aim 1) and cellular (Aim 2) mechanisms of DC-ASGPR-induced alloantigen-specific tolerance and the effectiveness of our novel anti-DC-ASGPR antibody in GVHD (Aim 3) and allograft rejection (Aim 4). At the end of this R01 study, we will understand the molecular and cellular mechanisms of DC-ASGPR-induced alloantigen-specific immune tolerance. The results of this study will have immediate implications for the rational design and development of novel immunotherapeutics for patients who undergo transplantation in the near future.